Bipolar electrodialysis (BPED) is a membrane separation process in which high purity acid and base solutions may be generated from a salt solution by the electrodialysis water splitting process. Typically, apparatus designed to perform such ED function consists of a stack containing a plurality of cation-selective membranes, bipolar membranes, and anion selective membranes positioned between a pair of electrodes. The stack may itself comprise an assembly of unit cells in which each unit cell comprises the above membranes arranged in such fashion to provide a plurality of flow paths or channels between adjacent membranes.
When a direct electrical current is applied to the bipolar membrane, water is split into OH− ions and H+ ions which migrate to the anode and cathode respectively. The cation selective membrane readily allows passage of the cations (positively charged ions, such as Na+, H+) therethrough while blocking passage of anions. Conversely, the anion selective membranes readily permit passage of the anions (negatively charged ions, such as Cl−, OH−) while retarding cation migration. If a salt solution such as NaCl is directed through the channel between the cationic selective and anionic selective membranes, the concentration of that salt solution is depleted with HCl and NaOH being formed in adjacent acid and base containing channels.
In connection with some prior art ED devices, H+ migration across the anion select membrane has proven problematic. This proton leak through the anion selective membrane is well known and is referred to as the Grotthuss mechanism by which protons diffuse through the hydrogen bonding network of water molecules. Anion selective membranes demonstrating substantial passage or migration of H+ are detrimental to electrodialysis processes, resulting in poor anion transfer current efficiency, low concentration of acid and base and high equipment and energy costs.